Tom Lane wrote:
> I'm wondering about doing something similar to what
> TransactionIdIsInProgress does, ie, make use of the PGPROC lists
> of live subtransactions. Suppose that GetSnapshotData copies not
> only top xids but live subxids into the snapshot, and adds a flag
> indicating whether the subxids are complete (ie, none of the subxid
> lists have overflowed). Then if the flag is set, tqual.c doesn't
> need to do SubTransGetTopmostTransaction() before searching the
> list.
Well, that sounds awfully more expensive than setting
local-to-my-database Xmins as well as global (all databases) Xmins :-)
On the other hand, ISTM as soon as one cache overflows, you have to go
check pg_subtrans which means the entire optimization buys nothing in
that case. It would be nice if the optimization degraded more
gracefully. I don't have any concrete suggestion though. The changes
proposed in the other CS-storm thread by the NTT person may help.
--
Alvaro Herrera http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support
